Add -buildroot patch and only build cython interfaces once.

Also:
- Drop LANGUAGES variable setting, now ignored by the sagemath build system.
- Drop unused SAGE_CBLAS variable from /usr/bin/sage.
- Do not force the C locale when launching sagemath.
- Allow the user to override SAGE_DEBUG in /usr/bin/sage.
- Add -ecm, -giac, and -latte patches to fix interactions with external tools.
- Add -sigfpe patch from upstream.
- Fix flask_autoindex and flask_openid imports in sagenb.
- Add Education category to the desktop file (bz 1624545).
- Fix SINGULAR_SO variable in /usr/bin/sage (bz 1636759 and 1655248).
- Invoke twistd-2 instead of twistd (bz 1640890).
- Improve jupyter integration (bz 1663165).
- Move existing jupyter integration into the notebook subpackage.
- Require python-jupyter-filesystem instead of owning its directories.
- Drop one more remnant of the F24 to F25 upgrade fixup.

sagemath-sagenb.patch

@@ -1,15 +1,22 @@
 diff -up build/pkgs/sagenb/src/sagenb/flask_version/base.py.orig build/pkgs/sagenb/src/sagenb/flask_version/base.py
 --- build/pkgs/sagenb/src/sagenb/flask_version/base.py.orig	2018-01-24 04:17:47.000000000 -0700
-+++ build/pkgs/sagenb/src/sagenb/flask_version/base.py	2018-08-29 08:44:38.823169061 -0600
-@@ -7,8 +7,6 @@ from functools import partial
++++ build/pkgs/sagenb/src/sagenb/flask_version/base.py	2019-01-18 15:02:10.691836714 -0700
+@@ -7,13 +7,11 @@ from functools import partial
  from flask import Flask, Blueprint, url_for, request, session, redirect, g, make_response, current_app, render_template
  from .decorators import login_required, guest_or_login_required, with_lock
  from .decorators import global_lock
 -# Make flask use the old session foo from <=flask-0.9
 -from flask_oldsessions import OldSecureCookieSessionInterface
  
- from flask.ext.autoindex import AutoIndex
+-from flask.ext.autoindex import AutoIndex
++from flask_autoindex import AutoIndex
  from sage.env import SAGE_SRC, SAGE_DOC
+ SRC = os.path.join(SAGE_SRC, 'sage')
+-from flask.ext.openid import OpenID
++from flask_openid import OpenID
+ from flask_babel import Babel, gettext, ngettext, lazy_gettext, get_locale
+ from sagenb.misc.misc import SAGENB_ROOT, DATA, translations_path, N_, nN_, unicode_str
+ from json import dumps
 @@ -28,7 +26,6 @@ class SageNBFlask(Flask):
      def __init__(self, *args, **kwds):
          self.startup_token = kwds.pop('startup_token', None)
@@ -68,7 +75,7 @@ diff -up build/pkgs/sagenb/src/sagenb/notebook/cell.py.orig build/pkgs/sagenb/sr
   
 diff -up build/pkgs/sagenb/src/sagenb/notebook/run_notebook.py.orig build/pkgs/sagenb/src/sagenb/notebook/run_notebook.py
 --- build/pkgs/sagenb/src/sagenb/notebook/run_notebook.py.orig	2018-05-22 10:16:28.000000000 -0600
-+++ build/pkgs/sagenb/src/sagenb/notebook/run_notebook.py	2018-08-29 08:44:38.824169048 -0600
++++ build/pkgs/sagenb/src/sagenb/notebook/run_notebook.py	2019-01-18 16:37:26.438289912 -0700
 @@ -53,12 +53,11 @@ sagenb.notebook.misc.DIR = %(cwd)r #We s
  # Flask #
  #########
@@ -86,3 +93,12 @@ diff -up build/pkgs/sagenb/src/sagenb/notebook/run_notebook.py.orig build/pkgs/s
  
  def save_notebook(notebook):
      print("Quitting all running worksheets...")
+@@ -336,7 +335,7 @@ reactor.addSystemEventTrigger('before',
+             profilecmd = '--profile=%s --profiler=cprofile --savestats'%self.profile_file(kw['profile'])
+         else:
+             profilecmd=''
+-        cmd = 'twistd %s --pidfile="%s" -ny "%s"' % (profilecmd, kw['pidfile'], conf)
++        cmd = 'twistd-2 %s --pidfile="%s" -ny "%s"' % (profilecmd, kw['pidfile'], conf)
+         return cmd
+ 
+     def get_old_settings(self, conf):